Seismic surveying is used to perform characterization of subterranean elements in a subterranean structure. Examples of subterranean elements of interest include hydrocarbon and mineral bearing structures, fresh water aquifers, gas injection zones, and other subterranean elements. Seismic surveying is performed by deploying seismic sources (e.g., air guns, vibrators, explosives, etc.) and seismic receivers (e.g., hydrophones, geophones, etc.). The seismic sources are used to produce seismic waves (e.g., acoustic waves) that are propagated into the subterranean structure, with some of these seismic waves reflected and or refracted from the subterranean elements of interest and received by the seismic receivers.
One type of seismic source is impulsive, such as dynamite or one or more air gun(s) for generating seismic signals. With an impulsive energy source, a large amount of energy is produced in a relatively short period of time. Another type of seismic source is a seismic vibrator that imparts a signal at a lower energy level but over a longer period of time.
A type of seismic vibrator employs a servo-hydraulic actuator mounted on a carrier vehicle and controlled by a control system to impart seismic signals into the earth. Such an actuator includes a mass that is supported on a fixed double acting piston. The piston is fixed to a baseplate that is held in contact with the ground. Movement of the actuator mass causes the baseplate to transmit energy into the earth.
During this operation the weight of the vehicle is supported on the baseplate (via isolation devices), so that the baseplate does not decouple from the surface. The movement of the actuator mass is controlled by an electrical drive signal, derived from a reference signal, both provided by a control system. The drive signal can be conditioned to cause the mass to move in either direction. The mass is driven via a torque motor and one or more hydraulic servo valve stages.
During operation of the vibrator the reference signal can be generated in the form of a sinusoidal wave that changes frequency at a controlled rate. Such a signal is termed a sweep and is characterized by start and end frequencies and fixed or variable rates of change of frequency and amplitude. Other forms of signals such as band filtered pseudo-random series may also be used.
In order to control the transmitted energy, various sensors mounted on the actuator provide signals that are used by the control system. In a common system, the drive signal operates a torque motor connected to the mass via two stages of servo hydraulics. With this system, changes in the level of the drive signal would change the acceleration of the mass and control of the output energy would be difficult. Sensor measurements of the displacement of the main valve and the displacement of the mass are used as feedback signals for control loops in the control system. The addition of these control loops allows mass displacement to be controlled, allowing easier control of the output energy. Conventionally, both these sensors are bipolar, in that they measure positive and negative displacements from a center zero.
Other sensors are used to estimate the energy output of the actuator and to allow the drive signal to be adjusted to make the output energy more closely match the reference signal in amplitude and phase, while minimizing distortions. Commonly, single or multiple accelerometers are mounted on both the mass and the baseplate or its supporting structure. These accelerometers are also bipolar, in that they measure both positive and negative acceleration.
As the sensors described above produce analog outputs and are bipolar, sensor measurement polarity can be reversed by a badly wired cable or connector. It is possible to replace a defective component or cable on a vibrator, and introduce a polarity reversal, such that the measured energy output appears to have the correct phase relationship to the reference, but in fact the actual motion of the actuator is anti-phase.
Conventionally, testing techniques are used to confirm the polarity of a seismic vibrator. While many different polarity conventions are possible, it is important to establish the direction of movement of the actuator mass when the controller provides a known stimulus as the reference signal. The components in the control system and actuator that either drive the mass or make measurements may be connected in one of two polarities. Testing the polarity of a seismic vibrator can be a time consuming process, is subject to human errors and usually has to be repeated regularly and following maintenance work on the seismic vibrator.